Igneous
Basalt
Mafic volcanic rock (primarily plagioclase feldspar and pyroxene)
Hardness: 6.0 on Mohs scale; Color: Dark grey to black, weathering to rust-brown; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8-3.0.
- Hardness
- 6
- Color
- Dark grey to black, weathering to rust-brown
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6.0 on Mohs scale; Color: Dark grey to black, weathering to rust-brown; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8-3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava (basaltic magma) at or very near the Earth's surface. Basaltic crust makes up most of the ocean floors and is found in vast 'traps' or flood basalt provinces across all geological ages.
Uses & applications
Extensively used in construction as aggregate for road base, concrete, and railroad ballast. Also used as dimension stone (tiles), in the manufacturing of stone wool insulation, and occasionally as structural 'basalt fiber' in engineering.
Geological facts
Basalt is the most common rock type in Earth's crust and also covers large areas of the Moon, Mars, and Venus. On the Moon, the dark areas known as 'maria' are actually vast basaltic plains formed by ancient volcanic eruptions.
Field identification & locations
Identify in the field by its heavy weight (density), dark uniform color, and very fine grains that are usually too small to see with the naked eye. It often breaks into sharp fragments or forms hexagonal 'columnar joints' in large outcrops. Common in volcanic regions like Hawaii, Iceland, and the Pacific Northwest.
